Recovery of rhenium and molybdenum values from molybdenite concentrates

ABSTRACT

An improvement in the recovery of molybdenum and rhenium values by roasting molybdenite which comprises preheating finely divided molybdenite concentrate and passing it downwardly through a vertically oriented reaction zone countercurrent to an upwardly traveling stream of high temperature oxygen, oxygen enriched air or oxygen-sulfur dioxide mixture heated by its passage through a roasting hearth at the bottom of the reaction zone. The rate of oxidation of sulfides is controlled by various means to keep the temperature in the reaction zone below that at which molybdenum oxide volatilizes with resultant inhibition of the volatilization of rhenium oxide. The process is attractive from the standpoint of pollution control as by-product sulfur dioxide gas ordinarily released to the atmosphere is produced in the exhaust gases in concentrations high enough to make its recovery economically feasible.

United States Patent 119] N.Y.; Milos Vojkovic, Libertyville, Ill.

[73] Assignee: Continental Ore Corporation, New

York, N.Y.

[22] Filed: Dec. 28, 1970 21 Appl. No.: 101,784

[52] 11.8. CI 75/1, 75/7, 75/84, 75/121, 423/53, 423/592 [51] Int. ClC221) l/02 [58] Field of Search 75/1, 2, 6, 7, 9, 75/21, 23, 26, 36, 84,121, 117; 423/592, 22, 53

[56] I References Cited UNITED STATES PATENTS 2,809,092 10/1957Zimmerley 75/121 X 2,345,067 3/1944 Osann 75/7 2,234,378 3/1941 Loring75/7 3,455,677 7/1969 Litz 75/117 3,117,860 1/1964 Bjerkerud.... 75/1213,458,277 7/1969 Platzke 75/121 3,196,004 7/1965 Kunda 75/121 MOLYBDENUMSULFIDE CONCENTRATE Lake et al. 1 Nov. 6, 1973 [54] RECOVERY OF NI D3,348,942 10/1367 gavenport 75/63 R M 3,579,328 5/1 71 as 75/531,970,467 8/1934 Mayr 75/1 [751 i 'lz tfi l fi g s f igg 383 FOREIGNPATENTS OR APPLICATIONS M'arcei colaenberg Tarrytow'm 1,265,486 3/1972Great Britain 75 1 Prirriary Examiner-L. Dewayne Rutledge AssistantExaminer-Peter D. Rosenberg Attorney-Sheridan, Ross & Burton 57 ABSTRACTin the reaction zone below that at which molybdenum oxide volatilizeswith resultant inhibition of the volatilization of rhenium oxide. Theprocess is attractive from the standpoint of pollution control asby-product sulfur dioxide gas ordinarily released to the atmosphere isproduced in the exhaust gases in concentrations high enough to make itsrecovery economically feasible.

14 Claims, 2 Drawing Figures PREHEATER OXYGEN FLA SH ROASTER MOLYBDICOXIDE CALCINE RE'ROASTER CALCINE LEACH AIR SOLUTION GASES TO GASESSCRUBBER ATMOSPHERE GASES SCRUBBER GASES SOLUTION COMPRESSOR SOLUTIONTECHNICAL GRADE MOLYBDIC OXIDE LIQUID OXYGEN SULFUR DIOXIDE UNDERFLOWSOLUTION WASH SCRAP WATER IRON COPPER CEMENTATION SOLUTION TO WASTE COPER PRECIPITATE AMMONIUM MOLYB DATE AMMONIUM PERRHENATE SOLUTION TOMOLYBDENUM AND RI-IENIUM RECOVERY PATENTEURUY 5 ms 3,770, SHEET 1 BF 2HIE . INVENTORS JAMES L. LAKE JOHN E L/TZ ROBERT B. COLEMAN MARCELGOLDENBERG MILOS VOJKOVIC ATTORNEYS RECOVERY OF RHENIUM AND MOLIIBDIENUMVALUES FROM MOLYBDENITE CONCENTRATES BACKGROUND OF THE INVENTION Thescarcity of rhenium in nature and its rapidly increasing importance inindustry emphasize the need for a highly efficient and economicalprocess for recover ing it from its ores. Molybdenum and rhenium areusually found together in the molybdenite (MoS mineral associated withthe so-called porphyry copper ore deposits. The molybdenite is usuallyseparated from the bulk of the copper minerals and is recovered as aflotation concentrate containing from 40% to 55% molybdenum. Then themolybdenum sulfide concentrate is roasted to produce an oxide productcontaining a minimum of sulfur, the chief objective of prior artprocesses being the ultimate recovery of molybdenum. The rhenium isvolatilized during roasting and generally was not recovered.

In presently used processes large volumes of air are passed through thesystem for temperature control and other reasons resulting in theby-product sulfur dioxide being contained in such large volumes ofexhaust gases, mainly air, that itsrecovery is not economically feasibleand it is released to the atmosphere creating serious presentdaypollution hazards. Sulfur dioxide gas ordinarily does not occupy muchmore than l2% of the volume of the exhaust gases in these processes.

A process directed to the recovery from molybdenite concentrate ofrhenium along with molybdenum is disclosed in US. Pat. No. 2,579,107 toBertolus. The present invention is directed to the recovery frommolybdenite of a high yield of rhenium either as a high gradeintermediate product or as a high grade metal along with a metallurgicalquality molybdic oxide calcine containing a minimum of copper and sulfurimpurities, all with an efficient use of heat and materials.

The normal practice for the recovery of rhenium from molybdeniteconcentrates consists of roasting the concentrate in air utilizing amultiple hearth-type roaster. A portion of the rhenium as rhenium oxideis volatilized and is collected as a fume with the dust in a scrubberwhere it is dissolved in water. Recoveries of rhenium from theconcentrate normally are in the 55% to 65% range. An authoratativeopinion by those skilled in the art is that a conventional plantoperated primarily for the recovery of rhenium in accordance with priorart procedure could effect a net recovery of 77% of the rhenium.

The samll concentration of rhenium oxide in the large volume of exhaustgas resulting from the use of air is an extreme disadvantage.

Accordingly, it is a principal objective of the invention to accomplisha high recovery of the molybdenum and rhenium content of molybdeniteconcentrates and to produce byproduct sulfur dioxide in the exhaust gasin high enough concentrations to warrant its recovery for use so that itwill not pass to the atmosphere as a polluting agent.

SUMMARY OF THE INVENTION In accordance with the process of the presentinvention up to 95% of the rhenium in molybdenite is consistentlyrecovered along with a high grade molybdic oxide calcine. A principalfeature of the process is the provision of a reaction zone in whichfinely divides molybdenite is effectively dispersed and moves verticallyand Countercurrent to an upwardly traveling stream of pure hot oxygen,oxygen-enriched air or an oxygensulfur dioxide mixture emanating from aconcentrate roasting furnace, the result being a highly effective oxidation of the sulfur content of the concentrate to sulfur dioxide, ofrhenium and molybdenum to their higher oxides, and volatilization ofrhenium oxide.

The process begins with roasting a preheated molybdenite concentrate inan oxygen or mixed oxygensulfur dioxide atmosphere in a vertical flashroaster. Sulfur dioxide is formed from the oxidation of sulfur and willmix with the oxygen introduced or it may be added with the oxygen tocontrol oxidation rate. The oxidizing gases and the molybdenum sulfideparticles pass countercurrently in the vertical section of the flashroaster to provide maximum surface contact between the particles andgases. The length of time of the concentrate particles in the verticalsection in countercurrent contact with oxidizing gases is regulated toinitiate the oxidation of each of the particles of molybdenum sulfide,such that each of the particles will have a layer of molybdenum trioxideon its outer surface. The particles pass downwardly through the verticalsection onto a rotating horizontal hearth where they are annealed for aperiod of time at approximately 550 to 680C. in an oxygen oroxygen-sulfur dioxide atmosphere. This annealing step permits completeoxidation of molybdenum to molybdenum trioxide. The major portion of therhenium is volatilized while the concentrate particles are roasted andannealed, and pass out of the roasting and annealing furnace with theoff-gases which contain the excess oxygen, the sulfur dioxide formed andadded during roasting, and a small quantity of dust which isincompletely roasted.

An improvement of the invention is the partial control of the rate ofoxidation of sulfides in the reaction zone and thereby the temperatureof the zone to keep it below that at which molybdic oxide volatilizes ormelts with resultant inhibition of the volatilization of rhenium oxide.Part of the heat is due to the oxidation of the sulfides, the rate ofthis oxidation being controlled by the gas stream composition and thestoichiometric ratio of available oxygen to concentrate in the reactionzone. This oxidation rate and the heat generated thereby are controlledby varying the stoichiometric ratio of oxygen to concentrate using theratio of sulfur dioxide to oxygen gases in the exhaust gas as a measureof this ratio existing in the reaction area. Other parameters affectingthe operation of the process may be varied, these being preheattemperature, height of reactor column and heat dissipation from thecolumn.

In the practice of the process it has been found that less than 10% ofthe molybdenum and up to of the rhenium report to the off-gas as fumesor dusts and are collected in a suitable scrubber.

BRIEF DESCRIPTION OF THE DRAWINGS The process will now be described withreference to the accompanying drawings in which:

FIG. 1 is a partial schematic sectional view of the combined roaster andvertical column comprising the flash roaster of the invention.

FIG. 2 is a flow diagram illustrating the operation of the flashroasting process of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT Referring to FIG. 1, the numeral 10indicates a conventional rotating hearth furnace used for roastingconcentrates. The furnace is illustrative of the type concentrateroasting furnace which is constructed to provide for the introductionand passage of hot gases over the material in the roasting area. Otherconventional furnaces of this type are moving horizontal conveyor(sintering machine), circular rabbled hearth (single or multiple),longitudinal rabbled'hearth (Edwards style roaster) and horizontalkilns. Beyond this feature of the invention is not limited to any typefurnace. A furnace in which a water cooling system is used fortemperature control is preferred. If the furnace is air-cooled, the airis not passed through the reaction zone of the flash roaster. Thefurnace may be operated under negative or positive pressure.

Although the illustrated furnace is of conventional construction and isrepresented partially in schematic, its principal operating parts willbe described. The furnace comprises the rotating hearth 12 rotatablysupported by wheels 14 on platform 16 which moves vertically along theinterior surfaces of main support legs 17. Central support member 18,which supports platform 16, is mounted for raising and lowering byhydraulic cylinder 20 to correspondingly raise and lower the rotatingfurnace 12 and the platform 16. The bed of the rotating hearth uponwhich the concentrate rests is indicated at 22. A water cooling systemfor the rotating hearth by which water enters the inlet 24 and leavesthrough the outlet 26 is indicated generally at 28. The furnace isprovided with the usual cutter assembly 30 to periodically break crustswhich form as the concentrate is roasted. Circumferentially-spacedinlets 32 permit the introduction of gases, such as oxidizing gases,into the hearth area formed by the bed of the hearth and roof 34. Thegases may be introduced under positive pressure, or reduced pressuregenerated at the outlet pipe 42. The hearth l2'rotates in liquid sealsindicated at 35 to provide a gastight hearth area. A.conventional driveunit 36 is provided for rotating the hearth. The structure justdescribed is well known in the art and forms no part of the inventionother than in combination with the structure now to be described.

In order to provide a vertically oriented reaction zone forcountercurrent contact of concentrate with gas, a hollow reactor column38 is mounted vertically on the furnace 10 so that its bottom end opensinto the hearth area as shown. The-vertical reactor column may beconstructed of refractory or insulated material or heat conductingmaterial provided with a cooling or heat transfer media. The reactorcolumn may be made vertically adjustable by constructing it in spoolsections or otherwise. For support, it is secured to the top of lbeam 39by means of a conventional flange and bolt arrangement as shown, thel-beam being attached to support 17. The outer casing of the verticalreactor column 38 forms an inner chamber 40 which is the reaction zoneof the vertical column. The reaction zone 40 is provided with an outlet42 for gases, fumes, dust, etc., including rhenium oxide. Since itsbottom end opens into the enclosed area above the rotating hearth 12,gas introduced into inlets 32 passes over the hearth bed 22 and upwardlythrough the reaction zone 40 and out the outlet 42.

A concentrate preheater unit 44 is mounted above the reaction zone 40 bysupports, not shown, and is connected to delivery pipe 46 so thatpreheated concentrate may be introduced downwardly from the pre- Thepreheater unit 44 is a conventional heater and need not be mounted abovethe reaction zone as heated concentrate from the preheater locatedelsewhere can be transferred to the upper end of delivery pipe 46.

The operation of the process of the invention in conjunction with theapparatus just described for a typical operation is as follows.

A finely divided molybdenite concentrate is introduced into thepreheater 44 in an inert atmosphere and brought to a minimum temperaturein the neighborhood of about 500C. At the same time, hot gas isintroduced into'the preheat port 56 and circulated through the reactionzone 40 as a start-up procedure. Oxygen gas may also be introduced atthis time through inlet ports 32 and circulated through the area aboverotating hearth bed 22 and out the outlet port 42.

When the reaction zone 40 has reached the required temperature,preferably between about 550 an d- The hearth bed 22 is rotating and thecalcine contin- I uously leaves the hearth through an outlet, not shown,preceded by a scraper, where it is collected and subsequently processedto recover metal values from it. The water cooling system 28 is used asnecessary to control the temperature of the material on rotating hearth22.

In the reaction zone 40, sulfur dioxide is formed by oxidation ofsulfides present in the molybdenite concentrate. This hot sulfur dioxidegas is mixed with the upwardly traveling hot oxygen and passes out theexit 42 into the scrubbers along with the other gases, fumes and dust.where some of it unites with water in the scrubbers to'form sulfurousacid and the remainder is collected. The sulfur dioxide producednormally occupies from about 30-50% of the volume of the exhaust gases.Excess oxygen which has not been consumed in the reaction zone passesthrough the outlet 42 and may be collected for recirculation through thesystem.

As is well known, the higher the oxide of rhenium the more soluble theoxide is in water, so that maximum oxidation of rhenium is desired forrecovery in the scrubbers. The higher oxides are more volatile. As isalso well known, rhenium oxide is formed by roasting rhenium sulfide inthe presence of oxygen at a temperature between 200 and 300C; however,this reaction does not readily proceed in the presence of molybdenumsulfide. After the major part of the sulfur has been driven off assulfur dioxide in the reaction zone, rhenium and molybdic oxides areformed in the temperature range of about 500 to 650C. The reaction zonetemperature during the oxidation of sulfides must be kept below that atwhich molybdic oxide volatilizes or melts with resultant inhibition ofthe volatilization of rhenium oxide. The rhenium oxide passes to thescrubbers while the molybdic oxide particles fall by gravity to therotating hearth 22. Some of the unoxidized sultides will also reach therotating hearth as well as impurities in the form of compounds ofcopper, iron, etc. Some of these latter impurities, along with somemolybdenum sulfide, also pass to the scrubbers.

The flow diagram of FIG. 11 gives a condensed showing of the processdescribed. After the scrubber solution containing molybdate andperrhenate ions leaves the thickener, the molybdenum and rhenium may beseparated and recovered from solution by conventional means. A preferredmethod is disclosed in co-pending patent application Ser. No. 94,268filed Dec. 2, 1970, now US. Pat. No. 3,681,016.

As shown in the flow diagram, the calcine from the roaster is leached toremove impurities and the leach solution filtered with technical grademolybdic oxide being recovered.

The efficiency of the system in converting a large percentage of themolybdenum and rhenium in the molybdenite to oxides is partiallyachieved through the high, exposed particle surface area at elevatedtemperatures which comes in contact with oxygen. This is promotedlargely by the efficient dispersion of the vertically traveling sulfideparticles and the maximum surface contact of the particles with thecountercurrent, upwardly traveling oxygen. Oxygen and preheatedconcentrate particles are introduced at rates to provide a preferablestoichiometric ratio of oxygen to metal surfides in the reaction zone of120% or more. An acceptable ratio was found to be about 170-240%. Thisratio can be as low as stoichiometric; however, the process proceedsquite slowly at stoichiometric and a much longer vertical column wouldbe required. Excessive amounts of oxygen beyond the above range could beused as oxygen can be recycled after separation of sulfur dioxide.

An atmosphere free of oxygen is maintained in the preheater at alltimes. Preferably an atmosphere of nitrogen is used. Preheating of theconcentrate before it reaches the reaction chamber greatly acceleratesoxidation between the hot sulfides and hot oxygen. ltalso shortens therequired time in the reaction zone 44]? for completion of the chemicalreactions involved and, accordingly, enables a shorter vertical columnto be used. This also reduces the volume of gas necessary in thereaction zone MD at all times.

Pure oxygen is used as the oxidizing medium; however, oxygen-enrichedair can be used but this creates the problem of removing introducednitrogen from the system. The more available oxygen per volume of gas,

oxidation reactions are occurring continuously through contact of oxygenwith the hot calcines on the rotating hearth. The gaseous oxidationproducts are carried by the oxygen up through the reaction zone and tothe scrubbers. These products are principally sulfur dioxide, rheniumoxides and molybdenum oxides.

Vertical orientation of the reaction zone is preferable in that it makesfeasible complete dispersion of the molybdenite concentrate particlesabove the reaction zone and before the particles enter the reactionzone. The oxygen updraft further disperses the falling particles. Asstated previously, the effective dispersion of the hot particles in thehot reaction zone gives maximum surface contact of the particles withthe oxygen to provide complete oxidation thereof. This feature isreferred to as flash roasting.

A further important feature of the invention is the use of sulfurousacid formed in the scrubbers for leaching of the molybdic oxideconcentrate in the recovery of the remaining rhenium from the molybdicoxide calcine. This feature contributes to the economy of the system.Other leaching agents may be used.

The efficiency of the system may be increased by reroasting the calcine.The molybdenum trioxide produced in the flash roaster may not becompletely oxidized and may contain some sulfur in addition to anycopper which may have been in the feed. The sulfur is more completelyoxidized by re-roasting the calcines in air at 600C. for thirty minutes.This second roast also insures almost complete oxidation of the copper.it was found most convenient to perform the reroast in an externallyheated kiln using a small air flow to complete the oxidation. Anaddition portion of rhenium is also volatilized during the re-roast andcan be collected by scrubbing the off-gas in a second scrubber.

The process described above consistently provides up to recovery ofrhenium from molybdenite concentrate and a molybdic oxide calcine ofincreased purity.

A refinement in the control of the process will now be described.

It is necessary to maintain proper temperature control for rheniumvolatilization while maintaining conditions that will prevent excessivevolatilization of molybdic oxide. The most efficient operation of thesystem is dependent upon the controlled rate of reaction and partialoxidation of each particle: of molybdenum sultide as it descends throughthe vertical column. This control is required to prevent completeoxidation in a confined zone which would result in high temperature andpossible volatilization or fusion of molybdenum oxide. The rheniumvolatilization maybe depressed because of the formation of anon-volatile oxide or. the possible surface sintering or fusion ofmolybdenum compounds which entrap the rhenium.

The oxidation process in the vertical column is to a degreeself-regulating by the variation occurring in the sulfur dioxide tooxygen ratio of the gas stream. As the molybdenum sulfide oxidizes, thesulfur dioxide concentration increases and the oxygen concentrationdecreases, resulting in a suppression of the reaction rate.

Accordingly, the ratio of sulfur dioxide to oxygen in the exhaust gasis, in effect, a measure of the efficiency of the process when the mostdesirable ratio is known. To demonstrate this, the process was operatedat various ratios of sulfur dioxide to oxygen in the exhaust gas, andhigh volatilization of rhenium oxide with high recovery of molybdenum inthe calcine occurred when operating at sulfur dioxide percentages byvolume of 30 and 35 .in the exhaust gas. These volumes are, of course,related to the ratio of sulfur dioxide to oxygen,

the latter being the only other gaseous component in the exhaust streampertinent to this control feature. The sulfur dioxide-oxygen ratio inthe exhaust stream can be partially controlled to provide the optimumvalue, if necessary, by the introduction of sulfur dioxide gas with theoxygen. The ratios reflected by 3035% volume of sulfur dioxide are by nomeans critical, but its use to provide favorable reaction zoneconditions illustrates the effectiveness of this method of control.

There are three other principal parameters affecting the temperaturecontrol and/or the oxidationvolatilization process, one or all of whichmay be used to control these factors in varying degrees. Theseparameters are: (l) preheat temperature, (2) the height of the reactorcolumn, and (3) heat dissipation from the column. The first of these,like the oxygen-sulfur dioxide ratio, is applied during the operation ofthe process. The latter two are built-in to the construction of theapparatus.

The preheat temperature is readily controlled by adjusting the heatinput to the indirect-fired preheat furnace.

The height of the reactor column determines the dwell time of thesulfide particles in the reaction zone for complete oxidation and forformation and volatilization ofrhenium oxide. The optimum height for agiven operation is developed by calculations and measurements derivedfrom pilot plant operation. For example, in a continuous pilot plantoperation excellent results were obtained using a vertical column 44inches in height and 6 inches in diameter with a rotating hearth 3 feetin diameter. These dimensional relationships are not critical and wouldchange with change in other variables, such as, concentratecharacteristics, composition of feed gases, rate of gas injection, etc.

The heat dissipated from the vertical column is controlled by design,and construction materials used. The construction can be varied'fromhighly insulated construction to high conductivity construction with acooling media. The radiation and convection loss of heat generated for ametal conducting material and a given feed rate can be readilycalculated. Additional heat may be removed from the column by watercooling or other heat exchange media.

' The results given below are illustrative of those obtained byapplication of the above-described process in conjunction with theapparatus described.

Table 1 shows some material balances obtained in roasting testsperformed on molybdenite concentrate.

TABLE l.ROASTlNG TEST RESULTS 650-750C. range 550-650C. range Preheattemperatures: Hearth temperature:

The data in Table 1 shows the variability of the rhenium content of theproduct produced at somewhat similar conditions. With careful control, amolybdenum product may be produced which contains only 5% of the feedrhenium, demonstrating that the process is effective to provide highyields of both molybdenum and rhenium.

The high sulfur dioxide content of the off-gas rapidly saturated thescrubber solution with sulfur dioxide. As shown in Table 2 below, about90% of the rhenium reporting to the scrubber was found to be soluble,whereas less than 20% of the molybdenum was soluble (see data in Table2). Therefore, under normal operating conditions of the process, atleast of the rhenium and less than 2% of the molydenum report to thescrubber solution. The insoluble portion of the dusts, containing theresidual molybdenum and rhenium, are separated from the scrubbersolution and recycled back to the roaster feed for retreatment.

TABLE 2.-BALANCE OF RHENlUM AND MOLYBDENUM .lN SCRUBBER SOLUTION ANDSOLIDS After the off-gas has been scrubbed of its loading of dust andfume, it consists mainly of sulfur dioxide and oxygen. The gas may bedried and compressed to liquefy the sulfur dioxide. The oxygen remainsin the gaseous form and is recycled to the flash roaster.

Table 3 shows the results from re-roasting a number of calcines:

TABLE 3.RE-ROASTlNG FLASH ROASTER CALClNES lN AlR AT 600C.

Feed calcine Product calcine Percent volatilized Re S Re S Sample (pp(pp Re S After re-roasting of the calcine, portions of the copper, theremaining rhenium not collected in the scrubbers, and a portion of thesulfur are soluble in mineral acid solutions. Since the process producesa dilute mineral acid -sulfurous acid in the scrubbers, it is used toleach the copper and the remaining rhenium from the calcine (Table 4).

About 7 percent of the molybdenum contained in the calcines is alsosolubilized in the sulfurous acid leach.

About 7% of the molybdenum contained in the calcines is also solubilizedin the sulfurous acid leach.

The leached residue is separated from the leach solution by filtrationand after drying is ready for packaging for sale. The leach solutionjoins the solutions from the scrubbers on the flash roaster andre-roaster.

The effectiveness of the above-described process is graphicallyillustrated by the high recovery of rhenium and molybdenum achieved. itprovides for the recovery of up to 95% of rhenium and high recovery ofmolybdenum in molybdenite with a minimum of process time and a minimumof oxygen and added heat. The economic advantages of these features areapparent. The process is adaptable to either a batch or continuousoperation.

it is an attractive side advantage of the process that a small volume ofexhaust gas containing a high percentage by volume of sulfur dioxide isproduced. The process is normally operated with an exhaust gas volumedischarge rate of 1,350 cubic feet per minute (CFM) with up to 220%excess oxygen and 50% by volume of sulfur dioxide in the exhaust gas.This high volume percentage of sulfur dioxide makes itsrecoveryeconomically feasible for various commercial uses. In contrast,present-day processes utilizing air for cooling and for supplying oxygenare of necessity operated with an exhaust volume discharge rate of40,000 CFM, 16 volume percent excess oxygen and 1-2 volume percent ofsulfur dioxide. This volume percentage of sulfur dioxide in the exhaustgas is so low that its recovery is not economically feasible because itinvolves processing such large volumes of gas. As a result the sulfurdioxide is exhausted to the atmosphere creating a serious pollutionproblem in heavily populated areas. The process of this inventioneliminates this problem.

The reduced volume of exhaust gas also results in a much higherconcentration of rhenium oxide in the exhaust gas than is obtained inconventional processes. As a result, recovery of substantially all ofthe rhenium is far more feasible and economical than in presentprosesses using air with resultant large volumes of exhaust gas to beprocessed for recovery of the rhenium oxide.

Reduction of the volume of gas processed through the system by a factorof about 3.0 results in adrastig reduction in the size of equipmentrequired with significant savings in equipment cost and floor space.

What is claimed is: i

ll. A method forrecovering rhenium and molybdic oxide from molybdeniteconcentrate which comprises:

a. rare-heating particles of said concentrate in an oxygen-freeatmosphere to a temperature not in excess of about "150C to raise thetemperature of the particles to promote flash oxidation of themolybdenite when the particles are introduced into a flash oxidationzone,

b. causing said preheated particles to fall through a first oxidizingzone of heated oxygen with said particles and heated oxygen movingcountercurrent to each other to disperse said pre-heatcd molybdeniteparticles in said heated oxygen to provide maximum particle surfacecontact with heated oxygen for effective oxidation, said first oxidationzone being heated substantially by the exothermic heat of the reactionsoccurring in said first oxidation zone,

c. controlling the temperature in the first oxidation zone during theintroduction thereto of said preheated particles and thereafter tomaintain a temperature therein above the voiatilization temperature ofrhenium oxide and below the volatilization temperature of molybdic oxideto form rhenium oxide, sulfur dioxide and molybdic oxide, which latteroxide along with other solids passes to a second oxidation zone whereany unoxidized molybdenite is completely oxidized, said second oxidationzone being heated by exothermic heat of the reactions occurring therein,

d. passing oxygen through said second oxidation zone to oxidizemolybdenite contained therein,

e. passing at least some of the oxygen travelling to said firstoxidation zone through said second oxidation zone to heat the oxygenbefore it reaches the first oxidation zone,

f. recovering rhenium oxide by collecting it in a recovery zone outsidethe first oxidation zone and dissolving it in water,

g. recovering rhenium from the water solution of rhenium oxide, and

h. recovering insoluble molybdic oxide from the second oxidation zone.

2. The process of claim 1 in which said concentrate is preheated toabout 500C.

3. The process of claim l in which molybdenum values are recovered.

4. The process in claim 1 in which rhenium values are recovered.

5. The method of claim 11 in which the temperature of the firstoxidation zone resulting from exothermic heat of reaction is controlledby controlling the reaction rate of the oxidation reactions occurringtherein.

6. The method of claim 5 in which said reaction rate is controlled byadjusting the relative feed rate of oxygen and molybdenite concentrateto the first oxidation zone to control the stoichiometric ratio ofoxygen to metal sulfides introduced therein.

7. The method of claim a in which said stoichiometric ratio is at leastone.

h. The method of claim 6 in which said stoichiometric ratio is at least9. The method of claim 6 in which sulfur dioxide is introduced to thefirst oxidation zone.

10. The method of claim 6 in which the sulfur dioxide-oxygen ratio inthe exhaust gases from the first oxidation zone is used to determine therelative rate of addition of oxygen and concentrate.

11K. The method of claim T in which the exhaust gas contains up to about50% by volume of sulfur dioxide.

12. The method of claim i in which the dwell time of concentrateparticles in the first oxidation zone is controlled by varying thediameter and height of said zone.

13. The process of claim l in which oxygen in the exhaust gases isrecycled for reuse in the method.

14. The process of claim 1 in which sulfur dioxide in the exhaust gasesis dissolved in water to form sulfurous acid and the sulfurous acid usedto leach impurities from the molybdic oxide calcine recovered from thesecond oxidation zone.

* =t= i= =t

2. The process of claim 1 in which said concentrate is preheated toabout 500*C.
 3. The process of claim 1 in which molybdenum values arerecovered.
 4. The process in claim 1 in which rhenium values arerecovered.
 5. The method of claim 1 in which the temperature of thefirst oxidation zone resulting from exothermic heat of reaction iscontrolled by controlling the reaction rate of the oxidation reactionsoccurring therein.
 6. The method of claim 5 in which said reaction rateis controlled by adjusting the relative feed rate of oxygen andmolybdenite concentrate to the first oxidation zone to control thestoichiometric ratio of oxygen to metal sulfides introduced therein. 7.The method of claim 6 in which said stoichiometric ratio is at leastone.
 8. The method of claim 6 in which said stoichiometric ratio is atleast 120%.
 9. The method of claim 6 in which sulfur dioxide isintroduced to the first oxidation zone.
 10. The method of claim 6 inwhich the sulfur dioxide-oxygen ratio in the exhaust gases from thefirst oxidation zone is used to determine the relative rate of additionof oxygen and concentrate.
 11. The method of claim 1 in which theexhaust gas contains up to about 50% by volume of sulfur dioxide. 12.The method of claim 1 in which the dwell time of concentrate particlesin the first oxidation zone is controlled by varying the diameter andheight of said zone.
 13. The process of claim 1 in which oxygen in theexhaust gases is recycled for reuse in the method.
 14. The process ofclaim 1 in which sulfur dioxide in the exhaust gases is dissolved inwater to form sulfurous acid and the sulfurous acid used to leachimpurities from the molybdic oxide calcine recovered from the secondoxidation zone.